A knee airbag device attached to a steering column is disclosed in patent document 1 below. To explain briefly, in this conventional technique, a knee airbag is stored in a folded state inside a column cover of a steering column, and in a frontal collision, the column cover develops in a double-door manner, and the knee airbag inflates and deploys in left and right directions in a radial shape that takes the steering column as an approximate center. Patent document 1: Japanese Patent Application Laid-Open (JP-A) No. 2002-37003